The present invention relates to actuators for the intake and exhaust valves of internal combustion engines, and specifically to an electronically actuable engine valve providing a signal indicating the valve position.
Electrically actuable valves allow improved engine control. Unlike valves actuated mechanically by cam shafts and the like, the timing on electrically actuable valves can be more freely varied during different phases of engine operation by a computer-based engine controller.
One type of actuator for such a valve provides a disk-shaped armature which moves back and forth between two cylindrical electromagnets. The armature is attached to the valve stem of the valve and is moved against the force of two opposing springs each positioned between the armature and an opposing core. In an unpowered condition, the armature is held in equipoise between the two cores by the opposing spring forces.
During operation, the armature is retained against one of the cores by a "holding" current in the retaining electromagnet. The spring between the armature and the retaining core is compressed while the other spring is stretched.
A change of state is effected, opening or closing the valve, by interrupting the current holding the armature in place. When this occurs, the energy stored in the compressed and stretched springs accelerates the armature off of the releasing core toward the opposing receiving core. When the armature reaches the receiving core, that core is energized with a "holding" current to retain the armature in position against its surface.
In a frictionless system, the armature reaches a maximum velocity at the midpoint between the two cores (assuming equal spring forces) and just reaches the receiving core assembly with zero velocity. In a physically realizable system in which friction causes some of the stored energy of the springs to be lost as heat, the armature will not reach the receiving core unless the energy lost to friction is replaced. This is accomplished by creating a "capture" current in the receiving coil which produces a magnetic force to attract the armature and pull it to the core. The capture current is necessarily initiated before the armature contacts the receiving core. Once the armature is captured by the receiving coil, the current can be reduced to a holding level sufficient to hold the armature against the core until the next transition is initiated.
Capture of the approaching armature requires that the capture current be of sufficient magnitude to draw the armature to the core. However, it is equally important that the speed at which the armature strikes the core be limited to prevent armature damage and/or core damage and to minimize impact noise. During valve closing, control of the capture current is necessary to limit valve-seating velocity and thereby to prevent valve and/or valve seat damage or premature valve wear and to minimize valve-seating noise. If the capturing current is turned on too soon (or is too great in magnitude), the armature may be accelerated into the core and the valve into its seat at excessive velocity. Conversely, the armature may not be captured by the receiving core and the valve may not close if the capture current is turned on too late (or is too low in magnitude). Therefore, it is important to know armature position and velocity as it approaches the receiving core to ensure that the capture current is initiated at the proper time or amount to ensure proper capturing of the approaching armature.
Electronic position sensors may be attached to the valve stem for this purpose. Unfortunately position sensors that are sufficiently accurate and robust enough to survive in the environment of an internal combustion engine are expensive and thus impractical.